The role of a thrust reverser when an airplane comes in to land is to improve the ability of an airplane to brake by redirecting forward at least some of the thrust generated by the turbojet engine. During this phase, the reverser blocks off the gas jet pipe nozzle and directs the stream ejected from the engine toward the front of the nacelle, thereby generating a reverse thrust which adds to the braking of the airplane wheels.
The means employed to achieve this reorientation of the flow vary according to the type of reverser. However, in all cases, the structure of a reverser comprises movable cowlings that can be moved between, on the one hand, a deployed position in which they open up within the nacelle passage intended for the deflected flow and, on the other hand, a retracted position in which they close off this passage. These moving cowlings may also perform a deflecting function or may simply activate other deflecting means.
In cascade-type thrust reversers, for example, the moving cowlings slide along the rails so that by moving back during the opening phase, they uncover cascades of deflection vanes located within the thickness of the nacelle. A system of link rods connects this moving cowling to blocking doors which deploy into the ejection duct and block the exit as a direct flow. In door-type reversers on the other hand, each moving cowling pivots so that it blocks the flow and deflects it and therefore takes an active part in this reorientation.
In general, these moving cowlings are actuated by hydraulic or pneumatic actuating cylinders which require a network for transporting any pressurized fluid. This pressurized fluid is conventionally obtained either by bleeding off the turbojet engine, in case of a pneumatic system, or by tapping off the hydraulic circuit of the airplane. Such systems require significant maintenance because the smallest leak in the hydraulic or pneumatic network will be difficult to detect and carry the risk of having consequences that are damaging both to the reverse and to other parts of the nacelle. Furthermore, because of the small amount of space available in the forward section of the reverser, installing and protecting such a circuit are particularly tricky and cumbersome.
To alleviate the various disadvantages associated with the pneumatic and hydraulic systems, thrust reverser manufacturers have sought to replace them and to fit as many as possible of their reversers with electromechanical actuators which are more lightweight and more reliable. A thrust reverser such as this is described in document EP 0 843 089.
However, electromechanical actuators also have a number of disadvantages that need to be overcome in order to take full advantage of the benefits they provide in terms of weight saving and smaller size.
In particular, electromechanical actuators entail the use of a complete electrical system comprising the actuators, power and control components, and sensors, it being possible for all of these components to exhibit failures.
It is standard practice, in the event of a failure of one of the components of this system, to make the system unavailable, since the thrust reverser cannot be used at the level of performance required by its specifications.